1. Field of the Invention
The invention relates to specimen holders that allow imaging and analysis and particularly to in situ holders capable of exciting a specimen for dynamic experimentation.
2. Description of the Prior Art
In situ is a Latin phrase meaning “in the place.” The ability to observe dynamic processes directly, close to their natural state as they undergo changes is vital for the advancement of research in many modern day applications. Various examples of in situ holders for transmission electron microscopy (TEM) include cooling holders, heating holders, nano indentation holders, straining holders, biasing holders and environmental cell holders. Holders are also utilized in a variety of other imaging and analysis devices.
Observations that require the presence of controlled environmental fluids around the specimen at elevated temperatures is one challenging aspect of in situ imaging and analysis. Fluids referred herein may include liquids, gases or plasma. An electron beam, such as that utilized by a TEM to create a specimen image, interacts strongly with matter. This leads to electron beam broadening which is detrimental to image resolution. To avoid extraneous scattering of the electron beam, it is desirable to have a very low-pressure beam path within the TEM column, optimally a high vacuum environment. A specimen immersed in fluids, opposes this basic principle. This limits the in situ environment height adjacent the specimen to less than a few hundred microns, making it extremely difficult to incorporate a heating system within the confined space of the holder necessary to create such an environment. Apart from this, high temperatures also give rise to specimen drift because of asymmetrical thermal expansion of the holder and consequential displacement of the specimen within the TEM. The presence of these fluids, therefore, decreases the resolution of the microscope images, limiting the amount of useful information that can be gathered.
In situ experimentation that requires the presence of fluids is carried out with the help of an environmental cell (E-Cell) that is part of the specimen holder. Typically, such a cell consists of two thin film windows, which completely seal a confined space around the specimen to provide a controlled atmosphere for in situ experimentation. The basic requirement for such a device is to contain the fluid within the cell so that the main microscope vacuum remains undisturbed.
Traditionally, heating holders employed resistive heating to elevate the specimen temperature. FIG. 1a and 1b illustrate the basic principle of resistive heater 100 utilized within a TEM specimen holder. Here, an electrical current is passed through a resistance coil 102, which is either wrapped around, as in FIG. 1a, or placed near the specimen cup 104, as in FIG. 1b, containing a TEM specimen 106 as shown. This generates heat within the coil, which in turn heats the specimen either by conduction, as in FIG. 1a or by radiation, as in FIG. 1b. This heating technology is highly constrained when used for in situ heating of a specimen within an environmental cell that contains fluids. The major limitations of these prior art systems include several factors. First, resistive heating requires a complex heating mechanism, which comprises many parts, including resistance coils, radiation shields, electrical connectors and insulating blocks. This complete assembly occupies a large volume. This technology, when incorporated within an in situ holder, would increase the fluid path length of the cell and dramatically reduce the image resolution of the specimen. Resistive heaters, furthermore, have a limited working life. The heating coils may last only a few hours, especially in the presence of gaseous oxidizing, reactive or reducing environments. Conduction and radiation from the heating coils not only heat the specimen but also the entire surrounding region of the environmental cell as well as the microscope goniometer or adjacent parts of other imaging or analytical devices. This introduces significant specimen drift, thereby limiting image resolution. It is also not possible to use resistive heating coils for the localized heating of the specimen.
Resistive heating is also limited by the maximum specimen temperature that can be achieved, generally limited to the range of 1,000° C. to 1,200° C. Moreover, the use of electrical current for heating can generate an electromagnetic field around the specimen, which may interfere with the electron imaging beam, limiting the image resolution and causing beam drift. Lastly, in light of the inefficient heating mechanism, the time required to attain steady state specimen temperatures can be significant. Most dynamic processes occur within a fraction of a second. This makes the use of resistive heating technology difficult for most modern imaging applications.
Some current TEM holders employ microelectromechanical systems (MEMS) which allow the imaging of the specimen at elevated temperatures as illustrated in FIGS. 2a and 2b. A MEMS-based system 120 utilizes two facing dies made from a silicon substrate; bottom die 122 and top die 124, with a central hole 126. Membranes 128, typically constructed of silicon nitride cover the central hole 126 of each die. These membranes form a gas-flow channel 129 with an inlet 130 and an outlet 132. This nanoreactor membrane contains an embedded heater 134 in the form of a thin platinum wire 136, as shown in FIG. 2b. The heating capability of up to 500° C. is derived from local electrical resistive heating of the platinum wire 136. The compact design of the MEMS based system provides a fluid path length of less than 10 microns and a stable and rapid specimen heating environment of less than one second.
There are, however, many shortcomings associated with such devices. The MEMS based TEM holders are designed to allow the imaging of small specimens such as particles. These holders cannot be used to observe a standard TEM specimen having a diameter of 3 mm. The specimen viewing area in a MEMS based holder is limited to a few square microns. In the MEMS based system the particle specimen is in contact with the silicon nitride membrane. This limits its use to a specimen temperature of less than 1,500° C. Lastly, the reaction chamber in a MEMS based holder is often limited to one time use.
One approach devised to carry out in situ gas flow experiments is the use of an environmental transmission electron microscope (ETEM). This is a term coined for TEM's modified to include a differentially pumped E-Cell. This ETEM consists of radial holes incorporated in the objective lens pole pieces for the first stage of differential pumping. The regular sample area of the ETEM is the controlled environment volume. Differential pumping systems are connected between apertures using appropriate vacuum pump technology. This permits higher gas pressure in the sample region, while maintaining high vacuum conditions in the remainder of the TEM column. A conventional reactor-type gas manifold system enables inlet of flowing gases into the ETEM, and a sample stage with a furnace allows samples to be heated.
The use of ETEM for in situ experiments has many disadvantages: (i) the high installation and operating cost of ETEM, especially to carry out only a dedicated set of experiments; (ii) ETEM can be used only to circulate gases over the specimen; (iii) since the ETEM does not have a completely sealed E-Cell, the gas pressure around the specimen may fail to replicate real life conditions, as it is necessary to observe gas-solid reactions at a specific atmospheric pressure; (iv) the gas path length within the ETEM is considerably large; (v) the gas circulation within the ETEM may contaminate the region around the objective lens pole pieces, which, apart from affecting the image resolution, could also affect the results of the next specimen analysis, which may require a different gaseous environment; (vi) it does not include a specimen heating system and heating has to be carried out using a standard heating specimen holder; and (vii) the partial pressure of any residual gas contained in the microscope column may negatively impact the experimental results.
Laser-induced breakdown spectroscopy (LIBS) is a type of atomic emission spectroscopy which utilizes a highly energetic laser pulse as the excitation source. LIBS operates by focusing the laser onto a small area at the surface of the specimen; when the laser is discharged it ablates a very small amount of material, in the range of picograms to nanograms, which instantaneously generates a plasma plume with temperatures of about 10,000-20,000 K. At these temperatures, the ablated material dissociates into excited ionic and atomic species. During this time, the plasma emits a continuum of radiation which does not contain any useful information about the species present, but within a very small timeframe the plasma expands at supersonic velocities and cools. At this point the characteristic atomic emission lines of the elements can be observed. O. Bostanjoglo and E. Endruschat, in “Kinetics of Laser-induced Crystallization of Amorphous Germanium Films”, Phys. Stat. Sol. (a), 91, 17 (1985), and H. Domer and O. Bostanjoglo, in “High-speed transmission electron microscope”, Rev. Sci. Instrum., 74 (10), 4369-4372, (2003) disclose a Q-switched Nd-YAG laser system attached to a TEM to investigate the crystallization of amorphous Ge films by time resolved microscopy. A. Takaoka, N. Nakamura, K. Ura, H. Nishi, and T. Hata disclose, in “Local Heating of Specimen with Laser Diode in TEM”, J. Electron Microsc., Vol. 38, No. 2, 95-100, 1989, heating specimens locally to a temperature greater than 1,000° C. by introducing a laser diode and small lens system into the vacuum space in the TEM. Some prominent laboratories have modified commercial TEM's by setting up an elaborate network of laser optics in order to pulse the electron beam as well as ablate the specimen. V. A. Lobastov, R. Srinivasan, and A. H. Zewail disclose, in “Four-dimensional ultrafast electron microscopy”, PNAS, Vol. 102, No. 20, 2005, a diode-pumped mode-locked Ti:Sapphire laser oscillator to develop a 4D ultra fast electron microscope. Here the laser is used to generate ultra fast electron pulses derived from a train of femtosecond pulses and concurrently heat the specimen and induce melting of metals. Similarly, T. LaGrange et. al., disclose, in “Single-shot dynamic transmission electron microscopy”, Appl. Phys. Lett., 89, 044105, 2006, the modification of a commercial JEOL2000 TEM and designed a dynamic transmission electron microscope (DTEM) with the help of an Nd-YAG laser system. This DTEM is used for vast arrays of applications including the in situ analysis of Nano wire catalysis and growth.
D. Shindo et. al., in “Development of a multifunctional TEM specimen holder equipped with a piezodriving probe and a laser irradiation port”, J. Electron Microsc., Vol. 58, No. 4, 245-249, 2009, disclose the development of a specimen holder to introduce laser irradiation onto the specimen to study various photo-induced phenomena. While this holder has the capability of introducing a laser beam onto the specimen, it does not have: (i) a provision for an E-Cell to observe the dynamic reactions between specimen and fluids; (ii) the ability to focus or adjust the laser beam and, most importantly, (iii) the ability to change the energy level, especially in the form of heat, of the specimen.
Many of these references highlight the importance of lasers in the field of imaging and analysis. They describe various forms of TEM's that have been modified to focus a laser beam onto the specimen. They do not, however, have any provision for a self contained specimen holder which permits the adjustment of the beam and the selective flow of fluids over the specimen in a controlled environment. Many of the references introduce the laser or other light source solely for the observation of the effect of the light source on the specimen. Moreover, the custom installation and operating cost of such modified TEMs are usually very high and the modifications are made to carry out very specific sets of experiments.
There remains a need, therefore, for an optimized in situ holder for the dynamic observation at elevated temperatures in the presence or absence of fluids. Such an in situ holder should have the capability of introducing a beam of electromagnetic radiation through the specimen holder and should be compatible with most major commercially available TEM's. It should be portable and should not involve any modification to the microscope for in situ microscopy.
The holder should accept a wide range of specimens, including a 3 mm diameter disk, particles dispersed on a grid or FIB lamellae contained on a support grid, and further should incorporate a compact heating design in order to minimize the fluid path length within the environmental cell. It should further provide the ability to heat the specimen in the presence of fluids to a temperature in excess of 2,000° C., while also providing the capability to heat a localized region of the specimen. This capability will also limit the amount of heat radiated and conducted from the hot specimen to the surrounding region of the environmental cell and microscope components, reducing specimen drift and minimizing the amount of energy required to reach the desired specimen temperature. Finally, it should provide the capability for thermal cycling of the specimen with a short time interval, while incorporating high steady state specimen temperatures in a small time duration.